This invention is directed generally to color television receivers, and more particularly to automatic color correction circuits for use in such receivers.
Modern television receivers commonly use some form of a color correction circuit, sometimes referred to as an "automatic tint control", to ensure that reproduced fleshtones appear to a viewer with the proper tint or hue. Some such circuits operate to change the phase of either the 3.58 megahertz reference signal and/or the 3.58 megahertz chroma signal to shift the phase of selected chroma signals toward fleshtone. Such phase-shifting type circuits typically include coils, capacitors and the like which cannot be fabricated on integrated circuits. Consequently, the use of such circuits in modern television receivers, which tend to be constructed using more integrated circuit chips is disadvantageous from a cost standpoint.
Several approaches have been proposed for correcting fleshtones by operating on chroma signals subsequent to their demodulation, thereby generally avoiding or at least reducing the number of coils and capacitors required for color correction. For example, U.S. Pat. No. 3,798,352 makes use of demodulated (B-Y) and (R-Y) color-difference signals for generating an error signal to modify the chroma signals. However, that disclosed structure requires variable resistors for accurately "setting up" the circuit. Because variable resistors are not readily fabricated on integrated circuits, its drawback is obvious for the reasons given above. In addition the range over which color correction occurs is sensitive to the amplitude of the color signals, thereby causing the correction range to be undesireably variable.
Another proposal for operating on demodulated chroma signals is disclosed in U.S. Pat. No. 3,749,825. According to this latter patent, demodulated (R-Y) and (G-Y) color difference signals are selectively combined to produce a correction signal which is applied to the demodulated (B-Y) color difference signal. However, the disclosed arrangement for developing the correction signal is somewhat sensitive to the D.C. quiescent voltages to which the color difference signals are reference. Moreover, accurate set-up of the disclosed circuit requires a variable resistor which is not readily fabricated on an integrated circuit.
U.S. Pat. No. 3,821,790 describes a color correction circuit which combines the outputs of the (R-Y) and (B-Y) demodulators so as to alter their relative gain and develop a phase-shift when color correction is desired. Such correction is based on varying the A.C. impedance in (R-Y) and (B-Y) amplifiers, which impedance is developed by at least one capacitor which cannot be readily fabricated by integrated circuit technology.
Another aspect which is common to the prior art discussed above is that those color correctors operate in an "open loop" mode as opposed to a "closed loop" or feedback mode. Consequently, the degree of correction which they provide tends to vary from unit to unit rather than giving predictable and reproducible correction results.
For the foregoing reasons, prior color correction circuits have not been entirely satisfactory and have been largely unadaptable for fabrication on integrated circuit chips. Hence, their inclusion in television receivers utilizing integrated circuit technology remains undesireable, at least from a cost standpoint.